Cadmium isotopic evidence for reduced deep-water marine primary productivity during the end-Permian mass extinction

Abstract

Marine primary productivity is a key control on the stability of marine ecosystems. Decreasing marine primary productivity might have led to the end-Permian mass extinction, although this is debated. However, the changes in primary productivity at different seawater depths during the end-Permian mass extinction remain poorly constrained. We investigated the Cd isotopic systematics of shallow-water (Dajiang) and deep-water (Shangsi) sedimentary sections in South China. Combined with previously published data from an intermediate–deep-water section (Meishan), we were able to reconstruct the depth changes in marine primary productivity. In the Dajiang section, prior to the mass extinction, the Cd isotopic compositions of seawater (δ114/110CdSW) varied from +0.73‰ to +0.92‰, with an average of +0.83‰±0.13‰ (2SD; n=11). During and after the mass extinction, δ114/110CdSW varied from +0.73‰ to +0.91‰, with an average of +0.81‰±0.12‰ (2SD; n=12). In the Shangsi and Meishan sections, prior to the mass extinction, δ114/110CdSW varied from +0.47‰ to +0.85‰, with an average of +0.66‰±0.16‰ (2SD; n=35). During and after the mass extinction, δ114/110CdSW varied from −0.19‰ to +0.36‰, with an average of −0.02‰±0.27‰ (2SD; n=27). During the end-Permian mass extinction, the marine primary productivity recorded by Cd isotopes in the relatively deep-water sections was considerably reduced, which may have caused the destruction of relatively deep-water marine ecosystems. We suggest that upward expansion of sulfidic and anoxic deep water, possibly due to the volcanic eruption of the Siberian Traps large igneous province, was one of the causes of the decrease in marine primary productivity. However, the marine primary productivity did not change in the shallow-water section during the end-Permian mass extinction, although changes in the types of marine primary producers are evident from the fossil record. This change in the types of primary producers may have contributed to the mass extinction in shallow-water platform environments. Vertical variations in the changes in primary productivity indicate that the environmental factors leading to the extinction event mainly began in deep waters during the end-Permian mass extinction.